Advantages and Challenges of Mid-latitude Super-Ensemble Coastal Water Level Forecasting

Abstract

In mid-latitude regions, year-round coastal water level forecast systems serve the public with reliable and consistent forecasts ranging from high-tide floods to extreme events. The Stevens Flood Advisory System (SFAS) is a probabilistic coastal water level forecast system forced by a super-ensemble of global atmospheric forecast products (Canadian, U.S., and European atmospheric forecast systems, termed sub-ensembles). We assessed two years of forecasts for the Mid-Atlantic/New York Bight region to demonstrate the advantages and challenges of these systems and determine how they can be improved. The accuracy of the SFAS ensemble-based central forecast and spread is compared to predictions using each sub-ensemble and NOAA’s Probabilistic Extra-Tropical Storm Surge (P-ETSS) forecasts. Averaged across 14 stations, the SFAS central forecast achieves a 1–20% lower Root-Mean-Square-Error (RMSE) on semidiurnal water level peaks than its sub-ensembles and 20–50% lower RMSE than P-ETSS. Assessing the probabilistic forecast spread, the Coverage of Observation by forecast area of Uncertainty (COU) of the SFAS 5th-to-95th percentile is close to the 90% optimal value, better than the spreads of P-ETSS and the sub-ensembles. Results show that model simulations and ensemble forecasts created with the highest resolution forcing result in the lowest error and bias. Analysis of the wind forcings for significant storm surge events reveals both underestimation and a counterclockwise wind direction bias over the inner continental shelf, both signatures of “coastal blurring” of land-atmosphere boundary layer winds in the model forcing. This leads to underestimation of along-coast winds and Ekman transport-driven storm surge, important for mid-latitude regions.